1. Field of the Invention
The present invention relates to multi-carrier communication systems, such as frequency division multiplexing (FDM) systems and orthogonal division multiplexing (OFDM) systems, transmitters, receivers and to methods of transmitting a signal in such systems.
2. Description of the Related Art
In a multi-carrier transmission system, such as FDM or OFDM, a single data stream is modulated onto N parallel sub-carriers, each sub-carrier signal having its own frequency range. This allows the total bandwidth (i.e. the amount of data to be sent in a given time interval) to be divided over a plurality of sub-carriers thereby increasing the duration of each data symbol. Since each sub-carrier has a lower information rate, multi-carrier systems benefit from enhanced immunity to impulse noise and reflections. A disadvantage of FDM systems is that a guard band frequency is required between each modulated sub-carrier to ensure that the spectrum of one sub-carrier does not interfere with the spectrum of another. The necessity for guard bands in an FDM system significantly lowers the spectral efficiency of the system.
Orthogonal frequency division multiplexing (OFDM) is a modulation technique that is based on FDM. An OFDM system uses a plurality of sub-carrier frequencies which are perpendicular in a mathematical sense so that the sub-carriers' spectra may overlap without interference. The orthogonality of OFDM systems removes the need for guard band frequencies and thereby increases the spectral efficiency of the system. OFDM has been proposed and adopted for many wireless systems. It is currently used in Asymmetric Digital Subscriber Line (ADSL) connections and in some wireless LAN applications, including WiMAX and IEEE 802.11 a/g. OFDM is often used in conjunction with channel coding, an error correction technique, to create coded orthogonal FDM or COFDM. COFDM is now widely used in digital telecommunications systems to make it easier to encode and decode such signals. The system has found use in broadcasting as well as certain types of computer networking technology.
FIG. 1 shows an example of a signal transmitted in a multi-carrier system such as OFDM. The signal comprises a sequence of time domain symbols (TDS) or symbol “blocks”, each time domain symbol comprising a set of N signals, the signals comprising pilot signals or data signals and being transmitted in parallel (i.e. at the same time) each by a different sub-carrier frequency. A sequence of time domain symbols forms an information unit F of the system. The information unit shown in FIG. 1 comprises 38 time domain symbols of which 32 TDS's are data symbol blocks (DSB) and 6 TDS's are pilot signal blocks (PSB).
In an OFDM system, a block of N modulated data source signals are mapped to N orthogonal sub-carriers by using an Inverse Fourier Transform algorithm (IDFT/IFFT) to form a block of N parallel signals, known as an “OFDM symbol”, in the time domain at the transmitter. Thus, an “OFDM symbol” is the composite signal of all N sub-carrier signals. An OFDM symbol can be represented mathematically as:
                                          x            ⁡                          (              t              )                                =                                    1                              N                                      ⁢                                          ∑                                  n                  =                  0                                                  N                  -                  1                                            ⁢                                                c                  n                                ·                                  ⅇ                                      j2π                    ⁢                                                                                  ⁢                    n                    ⁢                                                                                  ⁢                    Δ                    ⁢                                                                                  ⁢                    ft                                                                                      ,                  0          ≤          t          ≤                      T            s                                              (        1        )            where Δf is the sub-carrier separation, Ts=1/Δf is symbol time interval, and cn are the modulated source signals. The sub-carrier vector in (1) onto which each of the source signals is modulated c ∈ Cn, c= (c0, c1. . . cN−1) is a vector of N constellation symbols from a finite constellation. At the receiver, the received time-domain signal is transformed back to frequency domain by applying Discrete Fourier Transform (DFT) or Fast Fourier Transform (FFT) algorithm.
As can be seen from equation (1), an OFDM time domain symbol consists of the composite of a number N of independent modulated signals which are transmitted simultaneously, in the time internal Ts, by the transmitter of an OFDM system to one or more receivers. There is therefore the potential for all N signals to be in phase and, consequently, for the peak value of the composite OFDM waveform to be the sum of the peaks values of all sub-carrier signals. When these N signals are added up with the same phase, they produce a maximum peak power that is N times the average power. The peak-to-average-power-ratio (PAPR) is defined as follows:
                    PAPR        =                                            max                              0                ≤                n                <                N                                      ⁢                                                                            x                  ⁡                                      (                    n                    )                                                                              2                                            E            ⁡                          [                                                                                      x                    ⁡                                          (                      n                      )                                                                                        2                            ]                                                          (        2        )            Where E denotes expectation/average value. The theoretical maximum of the PAPR for N number of sub-carriers is 10 log(N) dB.
The high PAPR inherent in OFDM systems causes problems when the signal has to be subject to power amplifiers. Specifically, the high PAPR values can drive the amplifier into a saturation region, where an increase in input drive level does not result in an increase in output level. This non-linearity reduces the efficiency of the amplifier.
A number of approaches have been proposed to reduce the PAPR of multi-carrier multiplexed signals, in particular of OFDM signals, including: amplitude clipping, tone reservations (TR), and interleaving. However, these previously proposed techniques have been found to introduce various other problems which negate the improvement seen in the PAPR such as an increase in the required transmit signal power, a reduction in the data rate of the system and an increase in bit error and complexity of the system. Two other previously considered techniques which have proved to be highly effective at reducing PAPR are selected mapping (SLM) and the use of partial transmit sequences (PTS). Both of these techniques involve adjustment of the phase of the sub-carrier signals comprised in a time domain symbol in order to optimise the composite OFDM waveform that is eventually transmitted. Selected mapping (SLM), is described in detail in R. W. Bauml, R. F. H. Fischer and J. B. Huber, “Reducing the Peak to Average Power Ratio of Multicarrier Modulation by Selected Mapping”, Electronics Letters, Vol. 32, No. 22, October 1996. The use of Partial Transmit Sequences in described in S.H.Muller and J.B. Huber, “OFDM with reduced Peak-to-Average Power Ratio by Optimum Combination of Partial Transmit Sequences,” Elect. Lett., vol. 33, no.5, February 1997, pp.368-69.
According to the SLM technique, a sequence of data source symbols which will comprise a time domain symbol, Cn {Cn=c0, c1, c2, . . . cn−1} is subjected to U different phase vectors Qu, to create a set of different, but equivalent (in term of data content), signal representations of the time domain symbol. Key parts of a basic SLM transmitter 18 are shown in FIG. 2. Each vector Qu consists of a sequence of N phase elements φ0, φ1, φ2. . . , φN−1, and, according to the SLM procedure, each element is applied to a different sub-carrier of the time domain symbol so that the phase of each signal is adjusted by a single phase element of the vector. The vector Qu, may be represented mathematically by:Qu=[ejφ0u, ejφ1u, . . . ejφN−1u], where u∈{0,1, . . . , U−1},Φnu∈(0,2π) for all U vectors.
A set of U phase vectors is preferably stored in a phase vector storage unit of the SLM block 18, wherein the sequence of phase vector elements comprised in a particular phase vector Qu is preferably be generated randomly. Thus, the SLM technique effectively randomises the phase of each sub-carrier so that when the sub-carriers are added together, the signals are less likely to be in phase with one another and the resultant envelope OFDM is flatter. The transmitter then selects the vector which results in the OFDM symbol having the lowest PAPR for transmission.
According to the SLM technique, the original OFDM data symbol can be recovered from the received signal rn by multiplying the conjugate of applied vector, Qu*, to the received signal. To do this, knowledge of the applied vector sequence is needed at the receiver. The present invention is concerned with this technical problem.
There are two main ways in which the identity of the applied vector may be ascertained by the receiver. The first involves sending information about the applied phase sequences to the receiver as side information, so that the receiver is subsequently able to recover the original data symbol from a calculation of rn Qu*. The side information is commonly in the form of index number which uniquely identifies each of the phase vectors Qu. However, this approach utilises bandwidth and increases signalling overhead. Whilst in practice a large number of U vectors leads to a better PAPR reduction, any improvement in PAPR must be weighed against the logarithmic increase in the signalling overhead that is required as the number of U increases. For example, if U=16, then log2(U)=4= the number of bits required per OFDM time domain symbol as side information. In this case, if an information unit, or frame, of the system consists of 32 OFDM symbols, then 32*4=128 bits are needed to be sent to the receiver in every information unit. FIG. 3 shows the possible PAPR reduction that may be achieved with SLM using 4, 8, 16 and 64 vectors in an OFDM system comprising 512 sub-carriers applied on QPSK modulation without coding.
Another technique has recently been proposed in a paper entitled “A blind SLM receiver for PAR-reduced OFDM”, by A.D.S. Jayalath and C Tellambura, Proceedings of IEEE Vehicular Technology Conference, pp 219-222, Vancouver, Canada, 24 to 28 Sep. 2002. The proposed technique employs a decoder in the receiver which performs a set of calculations for each received OFDM data symbol. According to this technique, the value, or index number, representing the optimal phase vector which is applied to the OFDM data symbol is not transmitted to the receiver. Rather, the decoder, which has prior knowledge of all possible vectors Qu, performs a set of trial calculations to find the minimum distance between rn Qu* (where Qu* is the conjugate of one of the phase vector elements) and Ĥncn, where Ĥn is an estimate of the channel impulse response and Cn is one of the constellation points of the used modulation scheme which is known at the receiver. The received signal rn, after the DFT demodulation at the receiver is given by:rn=HnPnejφnu+nn  (3)Assuming a distortion less and noiseless channel, we can assume that the receiver gets a received signal vector r=c⊕Qu. Conceptually, for each received signal rn (n=0, . . ., N−1) the decoder computes the difference between each of a set of “trial” received signal and a representation of the actual received signal. If the trial received signal is correct, the difference will be zero, however in reality, the correct phase vector can be identified as the trial vector which results in the minimum difference. The decoder therefore computes the following decision metric for each time domain symbol received from the transmitter.
                    D        =                              min                          Q              u                                ⁢                                    ∑                              n                =                0                                            N                -                1                                      ⁢                                          min                                                      c                    n                                    ∈                  Mod                                            ⁢                                                                                                                                    r                        n                                            ⁢                                              ⅇ                                                                              -                            j                                                    ⁢                                                                                                          ⁢                                                      ϕ                            n                            1                                                                                                                -                                                                                            H                          ^                                                n                                            ⁢                                              c                        n                                                                                                              2                                                                        (        4        )            
A block diagram of such a receiver is shown in FIG. 4. The proposed scheme works on the assumption that (1) cn's are restricted to the constellation points of a particular modulation scheme, for example, QPSK; (2) the set of applied vectors is fixed and known at the receiver and (3) c⊕Qu and c⊕Qv are sufficiently different for u≠v. In other words, the set of available phase vectors have large Hamming distances, providing inherent diversity which can be exploited at the receiver.
The proposed blind receiver is advantageous in that the need to send side information is averted. However, the improvements seen in the throughput of the system are countered by a significant increase in receiver complexity. The signal processing required by the receiver is substantial and amounts to U*N*Ndata*M operations per information unit, where M refers to the number of constellation points for a particular modulation scheme, (e.g. QPSK) U is the number of possible vectors, N is the number of sub-carriers and Ndata is the number of OFDM time domain symbols. Another disadvantage of the known blind receiver is that for each OFDM symbol, the same modulation scheme should be used on each sub-carrier so that a mixed modulation scheme employing, for example, QPSK on some sub-carriers and 16QAM on other sub carriers, is not permitted. Furthermore, the receiver requires an accurate estimation of the channel where a high level modulation scheme is employed (16/64 QAM) which may mean that more pilot overhead is required.